Skin effect rf bridge filter

ABSTRACT

A filter in a bridge configuration having different types of iron coated conductors. The device excludes current from a load by utilizing the skin effect to attenuate RF energy. The bridge is unbalanced at low frequencies allowing low frequency energy and direct current to pass relatively unhindered.

United States Patent Schlachter 5] Nov. 28, 1972 [54] SKIN EFFECT RFBRIDGE FILTER 3,191,132 6/1965 Mayer ..333/79 72 Inventor: Dohald A.Schlachter 222 Braehead 3,329,911 7/1967 Schlicke et a1. ..333/79 D iFredericksburg, V3 22401 2,238,915 4/1941 Peters et 31 ..333/793,227,974 1/1966 Gray ..333/79 [22] Filed: April 20, 1971 [21] Appl.No.: 135,731 Primary Examiner,I-Ierman Karl Saalbach AssistantExaminer-Saxfield Chatmon, Jr. 52] US. Cl. ..333/75, 333/79, 333/74 sand wamn [51] Int. Cl. ..H03h 7/10, H03h 9/00 581 Field of Search..333/74, 75, 79; 332/51 [57] ABSTRACT 1 A filter in a bridgeconfiguration having different [56] v References C'ted types of ironcoated conductors. The device excludes UNITED STATES PATENTS currentfrom a load by utilizing the skin effect to attenuate RF energy. Thebridge is unbalanced at low 3,425,004 1/ 1969 Warner ..333/79frequencies allowing low frequency energy and direct 3,435,259 3/1969Mette ..332/51 x current to pass relatively unhinderm 3,309,633 3/1967Mayer ..333/79 3,125,733 3/1964 Holinbeck ..333/79 15 Claims, 3 DrawingFigures Pmmtnuovz m2 3.704.434

FIG.

r INVENTOR. pom/.0 A. sauna/rm JJMM 0.41%}.

ATTORNEY I SKIN EFFECT RF BRIDGE FILTER STATEMENT OF GOVERNMENT INTERESTBACKGROUND OF. THE INVENTION The present invention relates to filtersand more particularly to filters for protecting electroexplosive devicesfrom RF energy.

Most low pass filters use combinations of resistive,

capacitive and inductive elements in circuit configurations other thanbridge circuits. Tee, el, and pi networks are most commonly used, as arelossy dielectrics in broadband applications. These filters areunacceptable for protecting electroexplosive devices from spurious RFenergy because their performance depends upon their ability to causeimpedance mismatch between generators and loads. Impedance mismatchcannot be assured with these filters because in protectingelectroexplosive devices the generator impedance cannot be specified.The impedance of spurious electromagnetic generators may be any valuefrom a few tens of ohms to megohms and will generally be complex.

Other known devices for protecting ordnance from RF radiation hazardsemploy 'an electrostatic shield around the load while providing a lowresistance path to the firing circuit wires or use a transformer with athin metal barrier to separate the primary and secondary windings andthus reduce the RF power transmission. While these devices are effectivethey suffer from the disadvantage of being bulky and more costly thanthe ordinary filters.

SUMMARY OF THE INVENTION The general purpose of this invention is toprovide a much simplified approach to a device which excludes currentfrom a load and thus can be used to protect electroexplosive devicesfrom damage by spurious RF energy. The novel filter is veryuncomplicated in construction and can be produced at a very low cost.The filter is in the form of a Wheatstone bridge or lattice networkwhich uses resistive elements composed of a metal conductor coated witha second metal. At a predetermined frequency the filter, by utilizingthe skin effect, becomes a balanced bridge and thus excludes currentfrom a load. By suitable construction the invention can exclude eitherhigh or low frequency current from a load.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide a filter which excludes current from a load at certainfrequencies.

Another object of the present invention is to provide a filter which canbe constructed to attenuate either high or low frequencies.

Still another object of the invention is to provide a low cost anduncomplicated filter which can protect electroexplosive devices fromspurious RF energy.

Yet another object of the present invention is to provide a filter forprotecting electroexplosive devices which can be made small enough to bean integral part of the firing mechanism.

Still a further object of the presentinvention is to provide a filter toreduce the detrimental effects of the rectifying action of RF arcs andglow discharges. 7 Other objects, advantages and novel features of theinvention will become apparent from the following detaileddescription ofthe invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates theskin effect RF bridge filter.

FIG. 2 shows a cross-section of the resistive elements used in the skineffect bridge filter. t

FIG. 3 illustrates the bridge elements disposed in a symmetricalfashion. t

DETAILED DESCRIPTION OF THE PREFERRED EMBODIM NT The invention is adevice which is designed toutilize the well known skin effect toattenuate RF energy.

The skin effect is a property which is known in the art but is usuallyonly incidental to a particular filter and not part of the design. Whenan alternating current flows in a conductor, the alternating magneticflux within the conductor produces an emf which is greatest at thecenter of the conductor and decreases toward the outer skin. Thisinduced emf causes the current density to be greater at the surface ofthe conductor and to decrease towards its center. As the frequency ofthe alternating current is increased, the current within the conductorbecomes increasingly concentrated toward the outer skin. This phenomenonis known as the skin effect. The skin effect is responsible for the factthat the resistance of a conductor increases continuously with frequencywhile its internal inductance decreases continuously. The change inresistance and internal inductance becomes very pronounced forfrequencies above a minimum value which depends on the conductors size,magnetic permeability, and conductivity. t

The skin depth is defined as the distance measured inwardly from thesurface of the conductor in which the current in the conductor willdecrease by l neper, i.e., the current density becomes l/e times thedensity at the surfaceof the conductor, where e is the natural logarithmbase. The skin depth is given by the formula: 6= VZIwuE where w is equalto 21rf and is the angular frequency;

,1. is the permeability of the material, and

E is the conductivity of the material.

If a metal conductor is coated with a second metal, then forsufficiently high frequencies the current in the coated conductor willbe confined primarily to the coating material due to the skin effect.When this is the case, there is a negligible difference in impedancebetween the coated conductor and a similar solid conductor made from thecoating material. The elimination of hazards of RF energy toelectroexplosive devices requires the protection of the electric firingcircuits from randomly induced energies at all frequencies above anominal frequency in the region of l0KI-Iz to IOOKI-lz FIG. 1 is aschematic of the resistance elements arranged in a Wheatstone bridge orlattice network configuration. Resistance elements R,, are composed of aferromagnetic coating on a high resistance core. Resistance elements R,are composed of a ferromagnetic coating on a low resistance core. E isasource of electrical energy and L is an electroexplosive device orother load.

The bridge is unbalanced at DC and low frequencies due to the differentlow frequency resistance of R and R,,. As the frequency is increased alarger fraction of the total current is confined to the coating by theskin effect. At frequencies for which the thickness of the coating isapproximately equal to the skin depth, the resistance as well asinternal inductance of the elements R and R become equal. Given externalsymmetry the bridge is then balanced, and current through the load iscut off.

FIG. 2 shows a cross section of the resistance elements R, and R,,. Inthe preferred embodiment R has a high resistance core 12 of No. 36Nichrome wire one inch long with an ironcoating l0. Nichrome is thetrade name for a nickel-chromium alloy wire. R has a I low resistancecore l4of No. 36 copper wire 1 inch long with an iron coating 10. Theresistance elements could be constructed by threading a conductor withina ferromagnetic tube. The ratio of the radius of the resistance elements16 to the skin depth 8 can vary over a wide range. For example a ratioof 5&1 is an acceptabl value.

The embodiment shown would have an efficiency of about 50 percent at lowfrequencies, and an attenuation of about 40db at high frequencies giventhat the effective resistances of the conductors at high frequencies areheld within 2% of equality. The attenuation is, of course, proportionalto the accuracy to which the four conductors are matched in effectiveresistance.

The ferromagnetic coating could also be nickel, cobalt or aferromagnetic alloy. An alternate material for the high resistance corewould be constantan or some other high resistance material. v

The advantage of using a ferromagnetic metal for the coating is thatthey have a high permeability and a skin depth shorter than paramagneticand diamagnetic solid 52% nickel-iron alloy wire for the high resistancearms R The low resistance arms are formed from a similar nickel-ironalloy having a copper core occupying about one-third of the total wirediameter. All four bridge arms are twisted together in a spiral to forma single cable about 2 feet long. The twist is intended to givemechanical strength and flexibility as well as to balance the circuit toexternal magnetic fields. This embodiment is useful to connect thefiring circuit contact on the aft end of a rocket motor to the electricsquib in the forward end. r

Other alternative constructions for the bridge filter are possible byselecting suitable materials for the core and coating and by varying thecoating thickness, bridge wire outer diameter, and the length of thebridge arms. The high resistance elements of the bridge could be irontubing or iron coated dielectric filaments. lrori alloys to vary oroptimize permeability, conductivity, and Curie point could be used, aswell as conductive ferrites toadapt the filter to higherr'esistanceyloads. In the latter case, the bridge elements R would beferrite tubes, the two low'resistance arms R having metal conductorsthrough the center of the tube. Also, with larger conductors, lowercutoff frequencies and higher efficiencies would be feasible and thefilter could be used in the form of an attenuating'cable or transmissionline.

With similar constructions, high pass and notch filters are alsopossibleFor instance, two Nichrome wires and two iron wires with equalDC resistances would be balanced at DC and low frequencies, but as theskin effect occurred the bridge would become unbalanced, passing highfrequencies. Also two or more filters could be connected in cascade tooptimized attenuation in certain frequency bands or optimize powerdissipation characteristics.

To enchance power dissipating capabilities, the

bridge elements could be embedded in a low melting.

point material with high heat of fusion. If this material is metallic,it could be arranged to short out when it melts, giving a fail safeproperty. A fusible series element would also accomplish this.

The filter could also be constructed to give a visible alarm indicationby encasing the conductors in a glass envelope. For this embodiment theelement heat to incandescence and the core materials should be selectedso that at the Curie temperature of the magnetic coating theresistivities of the core materials are equal, maintaining balance eventhough the skin effect is reduced. This technique would be useful alsoto balance the bridge for DC and low frequencies at a critical RF level,so that the DC componentoof RF arcs would not present a hazard. Platinumand constantan should do this at about 900 Centigrade, as well as ironand Nichrome at some other temperature.

Thus there has been disclosed a skin effect bridge filter which has theadvantages of simplicity of construction, low cost and is capable ofproviding reasonable DC efficiency while maintaining high RFattenuation. Obviously many modifications and variations are possible inthe light of the above teachings.

What is claimed is:

l, A skin effect filter for excluding current from a load comprising: e

a load from which high frequency current is to be excluded; and

a plurality of resistive elements connected in the form of a Wheatstonebridge with said load connected to diagonally opposite junctions of saidbridge;

each of said resistive elements comprising a central core of conductivematerial coated with a second conductive material;

the central core of a first two opposed arms of said bridge having ahigher resistance than the centra core of a second two opposed arms;

whereby the bridge is unbalanced at low frequencies and, due to the skineffect, balanced at high frequencies to exclude current from said load.

2. The filter of claim 1 wherein the first two opposite resistiveelements have a central core comprised of a nickel and chromium alloy;and

the second two opposite resistive elements have a central core ofcopper.

3. The filter of claim 2 wherein the thickness of the conductive coatingis equal to the skin depth.

4. The filter of claim 3 wherein the conductive coating is aferromagnetic material.

5. The filter of claim 3 wherein the ferromagnetic material is iron.

6. The filter of claim 3 wherein the ferromagnetic material coating isnickel.

7. The filter of claim 3 wherein the ferromagnetic material is cobalt.

8. The filter of claim 1 wherein the resistive elements of the first twoopposite arms are comprised of ferrite tubes; and

the resistive elements of the second two opposite arms are comprised offerrite tubes having a metal conductor through the center.

9. The filter of claim 1 wherein the resistive elements are constructedto have an equal resistance above a frequency of about lOKHz.

10. The filter of claim 8 wherein the load is an electroexplosive devicewhereby the filter provides protection from randomly induced RF energy.

'11. The filter of claim 1 wherein the first two op- 12. The filter ofclaim 1 wherein the resistive elements of the first two opposite armsare solid conductors; and

the resistive elements of the second two opposite arms have an outerportion of the same material as the first two opposite arms and core ofa second conductive material.

13. The filter of claim 12 wherein the resistive elements of the firsttwo opposite arms are comprised of a nickel-iron alloy.

14. The filter of claim 13 wherein the core of the second two oppositearms have a core comprised of copper.

15. The filter of claim 1 wherein the core materials for the respectivearms are selected to maintain a balanced bridge at an increasedtemperature.

1. A skin effect filter for excluding current from a load comprising: aload from which high frequency current is to be excluded; and aplurality of resistive elements connected in the form of a Wheatstonebridge with said load connected to diagonally opposite junctions of saidbridge; each of said resistive elements comprising a central core ofconductive material coated with a second conductive material; thecentral core of a first two opposed arms of said bridge having a higherresistance than the central core of a second two opposed arms; wherebythe bridge is unbalanced at low frequencies and, due to the skin effect,balanced at high frequencies to exclude current from said load.
 2. Thefilter of claim 1 wherein the first two opposite resistive elements havea central core comprised of a nickel and chromium alloy; and the secondtwo opposite resistive elements have a central core of copper.
 3. Thefilter of claim 2 wherein the thickness of the conductive coating isequal to the skin depth.
 4. The filter of claim 3 wherein the conductivecoating is a ferromagnetic material.
 5. The filter of claim 3 whereinthe ferromagnetic material is iron.
 6. The filter of claim 3 wherein theferromagnetic material coating is nickel.
 7. The filter of claim 3wherein the ferromagnetic material is cobalt.
 8. The filter of claim 1wherein the resistive elements of the first two opposite arms arecomprised of ferrite tubes; and the resistive elements of the second twoopposite arms are comprised of ferrite tubes having a metal conductorthrough the center.
 9. The filter of claim 1 wherein the resistiveelements are constructed to have an equal resistance above a frequencyof about 10KHz.
 10. The filter of claim 8 wherein the load is anelectroexplosive device whereby the filter provides protection fromrandomly induced RF energy.
 11. The filter of claim 1 wherein the firsttwo opposite resistive elements are comprised of a nickel and chromiumalloy wire; and the second two opposite resistive elements are comprisedof iron wire having a DC resistance equal to the first two oppositeresistive elements whereby the bridge filter is balanced at lowfrequencies and unbalanced at high frequencies.
 12. The filter of claim1 wherein the resistive elements of the first two opposite arms aresolid conductors; and the resistive elements of the second two oppositearms have an outer portion of the same material as the first twoopposite arms and core of a second conductive material.
 13. The filterof claim 12 wherein the resistive elements of the first two oppositearmS are comprised of a nickel-iron alloy.
 14. The filter of claim 13wherein the core of the second two opposite arms have a core comprisedof copper.
 15. The filter of claim 1 wherein the core materials for therespective arms are selected to maintain a balanced bridge at anincreased temperature.